1. Field of the Invention
This invention relates to modems and to methods for encoding data for transmission through an analog communication system.
2. Description of Related Art
Frequency responses in analog communication systems may limit the rate or quality of information transmissions. For example, current telephone networks often employ transformers that have a frequency null at 0 Hz (i.e. for a DC signal component) and strong attenuation and non-linear distortion for signal components having frequencies less than about 100 Hz. Accordingly, signals having low frequency components that carry information are subject to information loss when transmitted through conventional telephone networks. Such losses can be a problem for modems that are connected through telephone networks.
A proposed 56 kbit/s PCM modem generates a digital PCM (pulse code modulated) signal having a sampling frequency of 8 kHz. The PCM signal is sent in digital form to a telephone system central office where the PCM signal is converted to an analog or PAM (pulse amplitude modulated) signal for transmission on a local loop. When generating the PCM signal, the PCM modem may use a one-to-one mapping from 7-bit data symbols to sample values in the PCM signal. For example, each sample value may represent and be equal to a 7-bit data symbol. For a conventional one-to-one mapping of uncorrelated digital data, the analog or PAM signal that results from digital-to-analog conversion of the PCM signal has a frequency spectrum that is essentially flat from 0 Hz up to a cut-off frequency of 4 kHz (i.e. half the sampling frequency). Accordingly, transformers in the telephone network can distort the information signal and degrade modem performance by attenuating the low frequency components relative to other components in the PAM signal.
Techniques have been proposed that change the spectrum of a PAM signal to reduce the DC component. A paper entitled "A Spectral Shaping Technique for PCM Modems," by Vedat Eyuboglu, that was submitted to the Telecommunications Industry Association (TIA), TR30.1 Ad-Hoc Group Meeting on PCM Modems on Nov. 13-15, 1996 proposed a mapping technique where the set of PCM sample values is larger that the set of data symbols, so that some data values can be represented by either of two alternative PCM sample values. The mapping, when possible, selects the one of the two alternative PCM sample values that minimizes the relative magnitude of the DC component of the analog signal so that transformers distort the PAM signal less. A second technique described in a paper entitled "DC Suppressor for 56K Modems," submitted by RSA Communications to the TR30.1 Ad-hoc Committee on Oct. 16, 1996 changes the sign of selected sample values in the PCM signal to minimize the DC component in the PAM signal. This technique adds overhead bits to indicate whether the signs of sample values have been changed so that a receiver can correct the signs if necessary when decoding data.
Even with these techniques, a communication systems may still distort the analog (or PAM) signal because the proposed mapping only compensates for variations in frequency response at low frequencies and other sources of variations in frequency response may be present. For example, a digital-to-analog converter that converts a PCM signal to a PAM signal may include a low pass filter that eliminates high frequency components that result from aliasing and the local loop may have a frequency response that suppresses or distorts the high frequency components. Thus, a telephone network may distort desired high frequency components of an information signal. An adaptable transmission process is desired that reduce the problems of high and low frequency distortions.